nearly all the houses were knocked down.

The igneous period of eruption having terminated, the ashes, lapilli,
and projectiles became more abundant, accompanied by thunder and
lightning. On the 28th they darkened the air, and the terrific noise of
the mountain continuing or increasing, the terror at Resina, Portici,
and Naples became universal. It seemed as though the tragic calamities
of the eruption of A.D. 79 were about to be repeated. But gradually the
force of the explosions decreased, and the noise from the crater became
discontinuous, so that on the 30th the detonations were very few, and by
the 1st May the eruption was completely over.

Such is a condensed account of one of the most formidable eruptions of
our era. In the frontispiece of this volume a representation, taken (by
permission) from a photograph by Negretti & Zambra, is given, showing
the appearance of Vesuvius during the final stage of the eruption, when
prodigious masses of smoke, steam, and illuminated gas issued forth from
the summit and overspread the whole country around with a canopy which
the light of the sun could scarcely penetrate.

It will be noticed in the above account that, concurrently with the full
moon, there were two distinct and special outbreaks of activity; one
occurring in March, the other in the month following. That the
conditions of lunar and solar attraction should have a marked effect on
a part of the earth's crust, while under the tension of eruptive forces,
is only what might be expected. At full moon the earth is between the
sun and the moon, and at new moon the moon is between the sun and the
earth; under these conditions (the two bodies acting in concert) we have
spring tides in the ocean, and a maximum of attraction on the mass of
the earth. Hence the crust, which at the time referred to was under
tremendous strain, only required the addition of that caused by the
lunar and solar attractions to produce rupture in both cases, giving
rise to increased activity, and the extrusion of lava and volatile
matter. It may, in general, be safely affirmed that low barometric
pressure on the one hand, and the occurrence of the syzygies (when the
attractions of the sun and moon are in the same line) on the other, have
had great influence in determining the crises of eruptions of volcanic
mountains when in a state of unrest.

_Contrast between the Northern and Southern Slopes._--Before leaving
Vesuvius it may be observed that th